Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 29, 2020	Mar. 31, 2019	Mar. 29, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 36,413	$ 55,944	$ 83,730	$ 97,976
Other comprehensive income, net of tax
Foreign currency translation adjustment, net of tax	(47,011)	9,245	(33,112)	(14,089)
Loss on cash flow hedge valuations	(7,365)		(5,695)
Loss on cash flow hedge valuations		(2,769)		(6,778)
Other comprehensive (loss) income attributable to Tetra Tech, net of tax	(54,376)	6,476	(38,807)	(20,867)
Other comprehensive (loss) income attributable to noncontrolling interests, net of tax	(4)	1	(2)	238
Comprehensive (loss) income attributable to Tetra Tech, net of tax	(17,979)	62,387	44,900	77,040
Comprehensive income attributable to noncontrolling interests, net of tax	12	34	21	307
Comprehensive (loss) income, net of tax	$ (17,967)	$ 62,421	$ 44,921	$ 77,347